Well, if we want to be optimistic - like I said, Bester must have known Ben-Zayn wasn't anywhere near stable enough to bring Sinclair down perminantly. I'm betting he was willing to settle for simply making Sinclair's life difficult for a few days, and wouldn't waste too much time punishing Grey - if he really cared about eliminating Sinclair, he'd have ensured a less neutral telepath was assigned to Ben-Zayn.